1. Field of the Invention
The present invention relates to an image processing apparatus and method thereof and, more particularly, to an image processing apparatus which decreases the tonal number of an image, and a method thereof.
2. Description of the Related Art
Image data having multi-tonal levels read by an image input apparatus such as a scanner and image data of graphics having multi-tonal levels generated or edited by a computer are represented by a display, printer, facsimile, digital copying machine, and the like. In this case, in order to save and transfer image data having multi-tonal levels, the data amount of image data is often reduced by decreasing the tonal number.
Various methods are available for conversion required to decrease the tonal number. For example, an error diffusion method and a minimized average error method nearly equivalent to the error diffusion method are popularly used as methods that realize preferred image quality.
However, the error diffusion method and minimized average error method suffer the following problems.
The first problem is delay of pixel generation. That is, generation of black pixels in a density region in which the density increases from a low density to intermediate and high densities (to be referred to as a rising part hereinafter) delays. Likewise, generation of white pixels in a density region in which the density decreases from a high density to intermediate and low densities (to be referred to as a falling part hereinafter) delays. Note that the delay of generation of pixels means a state in which when, for example, pixel values are checked in a raster-scan order, image data having multi-tonal levels have transited from a low density region to intermediate and high density regions, but transition of corresponding converted image data from the low density region to the intermediate and high density regions is not complete yet. Likewise, the delay means a state in which image data having multi-tonal levels have transited from a high density region to intermediate and low density regions, but transition of corresponding converted image data from the high density region to the intermediate and low density regions is not complete yet.
The second problem is reproducibility of a fine line. That is, a line which has a small density difference from a background region and has a small width may often be represented while being broken in the middle of it (to be referred to as “break” hereinafter) or may not be represented (to be referred to as “vanishing” hereinafter).
These problems occur since an error to be diffused adversely affects surrounding pixels. In order to solve these problems, Japanese Patent Laid-Open No. 2002-374412 sets a binarization threshold value T by:T(x,y)={I(x,y)×(K−1)+128}/K  (1)where I(x, y) is the pixel value of an original image at a pixel position (x, y), and
K is an arbitrary natural number equal to or larger than 2.
The method of uniquely setting the binarization threshold value T(x, y) from the data I(x, y) of the original image can provide a certain effect for delay of pixel generation, but it is insufficient to guarantee good reproducibility of a fine line. Hence, a break or vanishing occurs.